So to my understanding, if you change firmware your App should reflect/list the change, mine did each time I changed my LaView to EZVIZ and back to LaView. The App should still have the same Login so no change there.
Once you have the RCA firmware running, I would go to the App, not sure which App you are using, and remove the Doorbell, then once that is done I would remove/delete your User account, this way the Doorbell is no longer associated to the Doorbell in the cloud and the new owner should be able to create/register the DB themselves as if the bought it new. In theory this should work and this is what Nelly's suggested when I chatted with them back 12/2020.
I know my family member sold their house but can't say if this worked, this is how he did it, he has been contacted by his Realtor or the new owners...But who knows if the got it working. You could try it yourself by creating a dummy account then doing the same process over again.
I will say Pete_c bought a used DB on eBay and it was still registered to someone else and EZVIZ (I Believe) would not release the account so I believe he sent back the DB. I think that is how it happened, maybe he will chime in.
HTH